A known kind of filling head comprises concentric cylindrical ducts, the inner of which provides a communication between a valve at the bottom of a supply hopper and the bag to be filled and the outer of which enables air-borne dust created within the bag during filling to be extracted.
After filling a bag some material is likely to remain within the ducts which can be dislodged into the environment when the bag is removed from the filling head. A solution to this hazard is provided by EP 0 490 607 which discloses doors which can be manually closed over the open lower ends of the ducts before removing a bag from the filling head. A problem with this arrangement, however, is the possibility of an operator failing to operate the doors correctly.